1. Field
Embodiments of the present invention relate to a hydraulic control method of a regenerative braking system for vehicles in which a point of time when regenerative braking torque starts to be reduced is predicted and hydraulic control is performed in advance prior to this point of time so as to improve braking performance.
2. Description of the Related Art
A hybrid fuel cell vehicle is a hybrid electric vehicle having two or more kinds of braking force using an existing engine and an electric motor (driving motor) as power sources, and selectively uses power of the engine and power of the electric motor according to load and velocity of the vehicle and converts remaining energy into electric energy using the electric motor, thereby achieving high fuel efficiency and low pollution.
Such a hybrid fuel cell vehicle is traveled by rotating driving wheels of the vehicle with the electric motor operated by electric energy. Here, it is a very important subject to efficiently use the electric energy to drive the electric motor. For this purpose, if a deceleration or brake order is given by a driver, the electric motor functions as a generator and electric energy generated from the electric motor is stored in an electric condenser. While the electric motor functions as the generator, braking force is applied to vehicle wheels and such braking force is referred to as regenerative braking force. Consequently, the total braking force applied to the vehicle wheels is the sum of regenerative braking force generated from the electric motor and hydraulic braking force generated from a hydraulic control device.
Therefore, in order to satisfy driver's braking intention, generation of only hydraulic braking force equal to a value, obtained by subtracting regenerative braking force corresponding to an amount of braking force, generated from the electric motor, from braking force required by the driver, is required.
FIG. 1 shows profiles illustrating a braking pattern of a conventional regenerative braking system under the condition that a velocity of a Continuously Variable Transmission (CVT) or planet gear type vehicle is reduced as time goes by.
As shown in FIG. 1, regenerative braking torque (regenerative braking force) is decreased as the velocity of the vehicle increases, and is increased as the velocity of the vehicle decreases and is rapidly decreased at a designated low velocity of the vehicle. Therefore, hydraulic braking force needs to be decreased in a section in which the regenerative braking torque (regenerative braking force) is increased, and hydraulic braking force needs to be increased in a section in which the regenerative braking torque (regenerative braking force) is decreased.
In such a regenerative braking system, a point of time when the generated regenerative braking torque must be removed occurs. Due to slow responsiveness of hydraulic braking force compared to responsiveness of regenerative braking force at this point of time, the total braking force, i.e. the sum of the hydraulic braking force and the regenerative braking force, is not uniform, and thus a deceleration change frequently occurs, as shown in FIG. 1.